This invention relates to an optical device and, more particularly, to an optical device having an optical wave guide produced in the presence of acoustic standing wave.
A typical example of the optical wave guide is an optical fiber. In order to confine light in the optical fiber, the optical fiber has a multilayer structure shown in FIG. 1. A core 1a is enclosed in a clad 1b, and the core 1a and the clad 1b are, by way of example, formed of quartz glass and compound glass, respectively. The quartz glass is larger in refractive index than the compound glass, and incident light L1 repeats the total reflection on the boundary between the core 1a and the clad 1b. As a result, the light L1 proceeds along the core 1a, and is radiated from the other end. The multilayer structure is achieved by a double crucible pulling down method and so on.
Another example of the optical wave guide is known as xe2x80x9cdiffusion type optical wave guidexe2x80x9d. The diffusion type optical wave guide has an elongated portion with a large refractive index by replacing an element of single crystal material with another element. FIG. 2 illustrates the diffusion type optical wave guide. A piece 2a of single crystal compound of LiNbO3 is available for the diffusion type optical wave guide, and Li-site of the single crystal material is replaced with H+ or Ti3+ in an elongated portion 2b indicated by hatching lines. The elongated portion 2b is higher in refractive index than the remaining portion 2e of the single crystal compound, and serves as a wave guide. Incident light L2 is propagated along the elongated portion 2b or the wave guide, and is radiated from the other end.
Another example of the optical wave guide is illustrated in FIG. 3. The optical wave guide is categorized in the thin film wave guide, and has a plane wave guide of active material grown by using a liquid-phase epitaxy. The thin film wave guide increases the energy density of optically pumped laser light/oscillation light generated in the active material, and, accordingly, improves the oscillation threshold and the slope efficiency or input-and-output characteristics.
FIG. 3 illustrates a thin film wave guide or a planar optical wave guide disclosed by D. Pelenc et. al. in xe2x80x9cHigh slope efficiency and low threshold in a diode-pumped epitaxially grown Yb:YAG wave guide laserxe2x80x9d, Optics Communications, vol. 115, 1995, pages 491 to 497. A plane wave guide 3a, which is indicated by hatching lines, is sandwiched between Yb-doped YAG 3b and 3c. Al3+ site is partially replaced with Ga3+, and the active material is epitaxially grown on the Yb-doped YAG substrate 3b so as to form the plane wave guide 3a. Yb-doped YAG with Ga3+ is grown on the plane wave guide 3a, and the plane wave guide 3a is overlain by the Yb-doped YAG layer 3c. 
Various optical devices have been developed, and several optical devices are known as xe2x80x9cacousto-optic devicexe2x80x9d. An interaction between optical material and an acoustic wave is known as an acousto-optic effect, and the acousto-optic effect is available for an optical device.
FIG. 4 illustrates an optical deflector analogous to the electro-acoustic element disclosed in Japanese Patent Publication of Unexamined Application No. 50-143547. The optical deflector has an electro-acoustic transducer 4a attached to a block 4b of optic material. The electro-acoustic transducer 4a generates an ultrasonic wave 4c. 
Laser light L3 is obliquely incident onto the block 4b, and is propagated through the block 4b. The transmitted light L4 is radiated from the block 4b. When the electro-acoustic transducer 4a is driven for generation of the ultrasonic wave 4c, Bragg reflection takes place due to the ultrasonic wave 4c due to an interaction between photon and phonon. If the electro-acoustic transducer 4a changes the frequency of the ultrasonic wave 4c, the laser light L3 is diffracted, and is radiated from the block 4b as indicated by L4xe2x80x2.
In this instance, the acoustic wave is applied as a progressive wave. If the acoustic wave does not serve as a progressive wave, the diffraction intensity is drastically decreased, because the interaction between the photon and phonon causes the diffraction to take place.
A surface acoustic wave is also available for an optical device. FIG. 5 illustrates an optical filter for changing the spectrum distribution of an incident laser light L5. The optical filter is analogous to an optical deflector with a comb-like electro-acoustic transducer disclosed in Japanese Patent Publication of Unexamined Application No. 62-257133. The filter comprises a block 5a of optical material, an optical wave guide 5b formed on the block 5a and a comb-like electro-acoustic transducer 5c. The comb-like electro-acoustic transducer 5c generates an ultrasonic wave 5d, and the laser light L5 is propagated in the optical wave guide 5b in such a manner as to cross the ultrasonic wave 5d. The laser light L5 is radiated from the other side as transmitted light L6. However, when the laser light L5 is interfered with the ultrasonic wave 5d, the ultrasonic wave 5d diffracts a predetermined frequency component L6xe2x80x2. If the electro-acoustic transducer 5c varies the intervals 5e of the ultrasonic wave 5d, the filter changes the diffracted frequency component.
The prior art optical wave guide device encounters a problem in the production cost. As described hereinbefore, the optical wave guide requires a refractive index higher than the other portion, and the higher refractive index is achieved by bonding different materials to each other, replacing an element of the optical material with another element, diffusing a dopant into an optical element or using a hetero-epitaxy. These modifying techniques are carried out on a part of the optical material, and a masking step and/or lithography is necessary for the selective modification. This results in a complicated fabrication process. The complicated fabrication process requires various kinds of apparatus such as, for example, a sputtering apparatus, a thin film growing apparatus, an etching apparatus, a cleaning apparatus and an annealing apparatus. Therefore, the prior art optical device with an optical wave guide is so expensive.
The second problem inherent in the prior art optical wave guide is poor reproducibility. A part of the optical material is converted to a high refractive index portion through a chemical reaction, and various parameters dominate the chemical reaction. It is impossible to exactly control all the parameters. For this reason, the reproducibility is poor.
The third problem is that the prior art optical wave guide can not be formed in all the optical materials. Some optical materials do not widely change the refractive index, and the crystal structure of another optical material is destroyed through the selective modification.
It is therefore an important object of the present invention to provide an optical device which has an optical wave guide inexpensive, reliable and formed in an optical material which is not available for the prior art optical wave guide.
To accomplish the object, the present invention proposes to partially change a refractive index of optical material by using an acoustic standing wave.
In accordance with the present invention, there is provided an optical device comprising a block of optical material, and at least one acoustic wave generator for creating an acoustic wave existing as a standing wave in the block, and the standing wave changes a refractive index of a part of the block through an acousto-optic effect so as to form an optical wave guide in the block.